Prismane structures

The three Dewar structures 5, 11 and 12 (Dewar benzene) are also considered to contribute to the resonance hybrid (according to valence bond theory, approximately 20% in total) and to the extra stability. Dewar benzene has now been prepared. It is a bent, non-planar molecule and is not aromatic. It gradually reverts to benzene at room temperature. The Ladenburg structure, prismane i6), is an explosive liquid. Dewar benzene and prismane are valence isomers of benzene. 

Figure 10.12 (a) Prismane structure of [Ge6 CH(SiMej)2)6l (for clarity only the ipso C atoms of the R groups are shown), (b) The tetracycio structure of [GegBugBrj] with only the ipso C atoms of the Bu groups shown, (c) The cubane structure of [Geg(CMeEt2>g] again with only the ipso C atoms of the t-hexyl groups shown. 

Bicyclo[2.2.0]hexadienes and prismanes are valence isomers of benzenes. These compiounds actually have the structures that were proposed for benzenes in the nineteenth century. Prismanes have the Ladenburg formula, and bicyclo[2.2.0]-hexadienes have the Dewar formula. Because of this bicyclo[2.2.0]hexadiene is often called Dewar benzene. On page 32 it was mentioned that Dewar formulas are canonical forms (though not very important) of benzenes. Yet, they also exist as separate compounds in which the positions of the nuclei are different from those of benzenes. 

Fe-S and Fe-Fe distances in [2Fe-2S], [3Fe-4S], and [4Fe-4S] clusters are all very similar 2.3 and 2.7 A, respectively. In the [6Fe-6S] prismane model cluster, however, there is an additional Fe-Fe distance at 3.7 A (Fig. 2). If a [6Fe-6S] cluster were present in the Fepr protein, then this longer Fe-Fe distance should he visible with extended X-ray absorption fine structure (EXAFS). As a consequence, EXAFS studies were carried out at the CCLRC Synchrotron Radiation facility in Daresbury, UK. The two Fepr proteins (those of D. vulgaris and D. desulfuricans), as well as a synthetic [6Fe-6S] cluster, were subjected to an EXAFS study. Low-temperature EXAFS... 

Prismane valency refers to the old model assuming the presence of a [6Fe-6S] cluster (7). Hybrid cluster refers to the 4Fe cluster found to be present in the crystal structure. 

Crystallographic data (.continued) for transition metal tetrafluorides, 27 98 for transition metal trifluorides, 27 92 Crystallographic disorder, nitrosyl groups, 34 304-305 Crystallography fuscoredoxin, 47 380 prismane protein, 47 232-233 Rieske and Rieske-type proteins, 47 92-109 Crystal radii, of various ions, 2 7 Crystals, 39 402 Crystal structure actinide metals, 31 36 copper-cobalt supetoxide dismutases, 45 ... 

Proof for the existence of benzene isomers in irradiated benzene has been obtained in several ways. These will not be discussed in detail, but they may be classified broadly as physical and chemical. Nuclear magnetic resonance has been used by Wilzbach and Kaplan to identify benzvalene.39 Prismane has also been identified by NMR and by vapor-phase chromatography. The Dewar form has been synthesized in several steps which start with ris-1,2-dihydrophthalic anhydride. Photochemically this compound yields bicyclo(2,2,0)hexa-5-ene-2,3-dicarboxylic aqid anhydride. This was followed by catalytic reduction and oxidative decarboxylation to give the Dewar form of benzene.39 The method of synthesis alone provides some basis for structure assignment but several other bits of supporting evidence were also adduced. Dewar benzene has a half-life of about 48 hr at room temperature in pyridine solution and its stability decreases rapidly as the temperature is raised. 

It should be noted that compounds of the prismane, Dewar and benzvalene type above have been produced nonphotochemically by the trimerization of /-butyl fluoro-acetylene to give structures 9-H.ss.58... 

Another extraordinarily strained polycyclic hydrocarbon that has been prepared in recent years is prismane (the Ladenburg structure for benzene, see Exercise 1-6). 

Despite this large favorable energy change, the prismane rearranges by way of intermediate structures (Equation 11.40) with the activation enthalpies indicated. Hence the activation enthalpy for the direct conversion (Equation 11.39) must be greater than 33 kcal mole-1. This barrier is remarkably high for a... 

Figure 7 shows the ORTEP drawing of the hexasilaprismane 1237. Its structural parameters are listed in Table 4. The crystal has a two-fold axis of symmetry. The skeleton has a slightly distorted prismane structure with two triangular units [Si-Si = 2.374-2.387 A (av. 2.380 A) and ZSi-Si-Si = 59.8-60.3° (av. 60.0°)] and three... 

Table 5 summarizes the observed structural parameters of prismanes (MfiRe, M = C, Si, Ge) together with the calculated values for R = H. The M—M bonds within and between the three-membered units are denoted by a and b, respectively. As calculated for R = H1314, a is shorter than b in prismane (Cfilk,39) and its derivatives CgRe (R = Me40... 

TABLE 5. Structural parameters of prismanes comprising group 14 element... 

FIGURE 17. Three isomers of Si2oH20 having the [10]prismane, pagodane, and dodecahedrane structures. Reprinted with permission from References 10. Copyright 1995 American Chemical Society... 

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